Legislation for a statutory register of lobbyists would be introduced by July, Downing Street has announced.

But a Liberal Democrat spokesman said that they had yet to agree to all parts of the package, adding to the impression of a rushed response to the scandal.

Mr Cameron had faced criticism over delays to reforming the rules after warning three years ago that lobbying would be the next big scandal to hit politics.

Although the idea of a register was part of the Coalition Agreement, it was not in the recent Queen’s Speech which meant it was not due to be introduced before 2014 at the earliest.

A joint investigation by The Telegraph and the BBC Panorama programme has exposed how an MP and peer were prepared to lobby Parliament in exchange for payments. The affair carried echoes of the scandal which dogged John Major’s administration in the Nineties. Under the proposed new laws, it is expected that only lobbyists who agree to regulations and sign up to a new register will be allowed to “lobby” politicians.

Separate laws to allow MPs to be recalled and face a by-election if they break parliamentary rules will not now be introduced for at least another year.

Lobbying reform has previously been delayed by questions of whether companies and charities should be regarded as lobbyists. Under the new plans, only those paid to lobby as “third parties” will be regulated.

Financial penalties would be imposed on any who refused to take part in the scheme, Downing Street said.

But the Conservatives are insisting that new laws also tighten rules on trade unions. They will close a loophole which allegedly allows unions to campaign at general elections but avoid the full force of a cap on what parties can spend.

Under the proposals, they will have to demonstrate annually that their membership figures are accurate. The change could affect Labour’s funding.

Labour sources called it “a shabby and panicked response by David Cameron to divert attention from a set of damaging headlines for the Conservative Party”. Many MPs believe tougher rules would not prevent a repeat of the cash-for-questions scandal, which was simply a case of politicians behaving unethically.

Last week, Patrick Mercer said he had resigned the Tory whip and would step down from Parliament at the next election after being exposed by this newspaper. He has also referred himself to the parliamentary watchdog

A legal expert on the new Bribery Act warned that the MP for Newark could face criminal prosecution.

“The indications are that Patrick Mercer and others will now face a police investigation into both bribery and misconduct in public office,” said James Carlton of Fox Williams LLP. “Mercer’s conduct may very well bring him under the scope of the new Bribery Act.”

The House of Lords standards commissioner began investigating three peers ensnared in the lobbying scandal. Lord Hill of Oareford, the leader of the Lords, described the allegations as “very serious and distressing”. Separately, Dan Byles, a Tory backbencher, will try to change the law so peers convicted of a criminal offence can be thrown out of the Lords.